Factor XII Assay

Definition

Factor XII assay is a blood test to measure the activity of factor XII -- one of the substances involved in blood clotting.

Alternative Names

How to prepare for the test

There is no special preparation needed for adults.

How the test will feel

When the needle is inserted to draw blood, you may feel moderate pain, or only a prick or stinging sensation. Afterward, there may be some throbbing.

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Why the test is performed

Your doctor may order this test if you had abnormal results on the partial thromboplastin time (PTT) blood-clotting test.

Normal Values

A normal value is 50-200% of the laboratory control or reference value.

What abnormal results mean

Decreased factor XII activity may indicate:

• Congenital deficiency of factor XII
• Liver disease

What the risks are

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)
• Multiple punctures to locate veins

Special considerations

Factor XII deficiency is not normally associated with symptoms of too much bleeding. In addition, factor XII does not appear to be necessary for the formation of clots during the normal process of stopping bleeding.

References

McPherson RA and Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 21st ed. Philadelphia, Pa.: W.B. Saunders; 2006.

Hoffman R, Benz Jr. EJ, Shattil SJ, et al., eds. Hematology: Basic Principles and Practice. 4th ed. Philladelphia, Pa: Churchill Livingston; 2005:1936.

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Review Date:2/26/2007
Reviewed By: William Matsui, MD, Assistant Professor of Oncology, Division of Hematologic Malignancies, The Sidney Kimmel Comprehensive Cancer Center at Johns Hopkins, Baltimore, MD. Review provided by VeriMed Healthcare Network.